Projection system for EUV lithograhphy

ABSTRACT

There is provided an EUV optical projection system. The system includes a first mirror, a second mirror, a third mirror, a fourth mirror, a fifth mirror, and a sixth mirror situated in an optical path from an object plane to an image plane, for imaging an object in said object plane into an image in said image plane. The image has a width W and a secant length SL, and the width W is greater than about 2 mm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a microlithography objective, aprojection exposure apparatus containing the objective, and a method ofmanufacturing an integrated circuit using the same. More particularly,the present invention relates to an optical projection system forextreme ultraviolet (EUV) lithography, particularly including sixmirrors arranged in two optical groups.

2. Description of the Related Art

It is widely accepted that current deep ultraviolet (DUV) projectionprinting systems used in a step and scan mode will be able to addressthe needs of the semiconductor industry for the next two or three devicenodes. The next generation of photolithographic printing systems willuse exposure radiation having soft x-ray or extreme ultravioletwavelengths of approximately 11 nm to 15 nm, also in a step and scanprinting architecture. To be economically viable, these next generationsystems will require a sufficiently large numerical aperture to addresssub 70 nm integrated circuit design rules. Further, thesephotolithography systems will require large fields of view in the scandirection to ensure that the throughput (defined in terms of wafers perhour) is sufficiently great so that the process is economically viable.

The theoretical resolution (R) of a lithographic printing system can beexpressed by the well-known relationship R=k₁λ/NA, where k₁ is a processdependent constant, λ is the wavelength of light, and NA is thenumerical aperture of the projection system. Knowing that EUV resistssupport a k₁-factor of ˜0.5 and assuming a numerical aperture of 0.20,an EUV projection system can achieve a theoretical resolution on theorder of approximately 30 nm with λ=13.4 nm. It is recognized in thepresent invention that all reflective projection systems for EUVlithography for use in a step and scan architecture having both a largenumerical aperture (0.20 to 0.30) and a large field (2 to 3 mm) aredesired to address the sub-50 nm linewidth generations as defined by theInternational Sematech's International Technology Roadmap forSemiconductors (1999).

Four-mirror projection systems, such as those described in U.S. Pat. No.5,315,629 and 6,226,346, issuing to Jewel and Hudyma, respectively, lackthe degrees of freedom necessary to correct aberrations over asufficiently large NA to achieve 30 nm design rules. The '346 patentteaches that a four-mirror projection system can be used to correctaberrations at a numerical aperture up to 0.14 (50 nm design rules).However, it is desired that the width of the ring field be reduced toenable wavefront correction to the desired level for lithography. The'346 patent demonstrates that the ring field is reduced from 1.5 mm to1.0 mm as a numerical aperture is increased from 0.10 to 0.12. Furtherscaling of the second embodiment in the '346 patent reveals that thering field must be reduced to 0.5 mm as a numerical aperture isincreased further to 0.14. This reduction in ring field width resultsdirectly in reduced throughput of the entire projection apparatus.Clearly, further advances are needed.

Five-mirror systems, such as that set forth in U.S. Pat. No. 6,072,852,issuing to Hudyma, have sufficient degrees of freedom to correct boththe pupil dependent and field dependent aberrations, thus enablingnumerical apertures in excess of 0.20 over meaningful field widths (>1.5mm). While minimizing the number of reflections has several advantagesparticular to EUV lithography, an odd number of reflections create aproblem in that new stage technology would need to be developed toenable unlimited parallel scanning. To “unfold” the system to enableunlimited synchronous parallel scanning of the mask and image withexisting scanning stage technologies, it is recognized herein that anadditional mirror should be incorporated into the projection system.

Optical systems for short wavelength projection lithography utilizingsix or more reflections have been disclosed in the patent literature.

One early six mirror system is disclosed in U.S. Pat. No. 5,071,240,issuing to Ichihara and Higuchi entitled, “Reflecting optical imagingapparatus using spherical reflectors and producing an intermediateimage.” The '240 patent discloses a 6-mirror catoptric or all-reflectivereduction system utilizing spherical mirrors. This particular embodimentis constructed with three mirror pairs and uses positive/negative (PIN)and negative/positive (NIP) combinations to achieve the flat fieldcondition. Ichihara and Higuchi also demonstrate that the flat fieldimaging condition (zero Petzval sum) can be achieved with a system thatutilizes an intermediate image between the first mirror pair and lastmirror pair. The patent teaches the use of a convex secondary mirrorwith an aperture stop that is co-located at this mirror. It is alsoclear from examination of the embodiments that the '240 patent teachesthe use of low incidence angles at each of the mirror surfaces to ensurecompatibility with reflective coatings that operate at wavelengthsaround 10 nm.

While the embodiments disclosed in the '240 patent appear to achievetheir stated purpose, these examples are not well suited forcontemporary lithography at extreme ultraviolet wavelengths. First, thesystems are very long (˜3000 mm) and would suffer mechanical stabilityproblems. Second, the embodiments do not support telecentric imaging atthe image which is desired for modern semiconductor lithography printingsystems. Lastly, the numerical aperture is rather small (˜0.05) leavingthe systems unable to address 30 nm design rules.

Recently, optical projection production systems have been disclosed thatoffer high numerical apertures with at least six reflections designedspecifically for EUV lithography. One such system is disclosed in U.S.Pat. No. 5,815,310, entitled, “High numerical aperture ring fieldoptical projection system,” issuing to Williamson. In the '310 patent,Williamson describes a six-mirror ring field projection system intendedfor use with EUV radiation. Each of the mirrors is aspheric and share acommon optical axis. This particular embodiment has a numerical apertureof 0.25 and is capable of 30 nm lithography using conservative (˜0.6)values for k₁. The '310 patent suggests that both PNPPNP and PPPPNPreimaging configurations are possible with a physically accessibleintermediate image located between the third and fourth mirrors. Thisparticular embodiment consists, from long conjugate to short conjugate,of a concave, convex, concave, concave, convex and concave mirror, orPNPPNP for short. The '310 patent suggests that both PNPPNP and thePPPPNP power distributions can achieve 30 nm design rules.

The preferred EUV embodiment disclosed in the '310 patent suffers fromseveral drawbacks, one of which is the high incidence angles at each ofthe mirrored surfaces, particularly on mirrors M2 and M3. In someinstances, the angle of incidence exceeds 24° at a given location on themirror. Both the mean angle and deviation or spread of angles at a givenpoint on a mirror surface is sufficient to cause noticeable amplitudeand phase effects due to the EUV multilayer coatings that mightadversely impact critical dimension (CD control).

Two other catoptric or all-reflective projection systems for lithographyare disclosed in U.S. Pat. No. 5,686,728 entitled, “Projectionlithography system and method using all-reflective optical elements,”issuing to Shafer. The '728 patent describes an eight mirror projectionsystem with a numerical aperture of about 0.50 and a six-mirrorprojection system with a numerical aperture of about 0.45 intended foruse at wavelengths greater than 100 nm. Both systems operate inreduction with a reduction ratio of 5×. Like the systems described inthe '310 patent, these systems have an annular zone of good opticalcorrection yielding lithography performance within an arcuate shapedfield. While these systems were designed for DUV lithography and arefine for that purpose, these embodiments make very poor EUV projectionsystems. Even after the numerical aperture is reduced from 0.50 to 0.25,the incidence angles of the ray bundles are very large at every mirrorincluding the mask, making the system incompatible with either Mo/Si orMo/Be multilayers. In addition, both the aspheric departure and asphericgradients across the mirrors are rather large compared to the EUVwavelength, calling into question whether or not such aspheric mirrorscan be measured to a desired accuracy for EUV lithography. Recognizingthese issues, the '728 patent explicitly teaches away from usingcatoptric or all-reflective projection systems at EUV wavelengths andinstead restricts their use to longer DUV wavelengths.

Another projection system intended for use with EUV lithography isdisclosed in U.S. Pat. No. 6,033,079, issuing to Hudyma. The '079 patententitled, “High numerical aperture ring field projection system forextreme ultraviolet lithography,” describes two preferred embodiments.The first embodiment that the '079 patent describes is arranged with,from long to short conjugate, a concave, concave, convex, concave,convex, and concave mirror surfaces (PPNPNP). The second preferredembodiment from the '079 patent has, from long to short conjugate, aconcave, convex, convex, concave, convex, and concave mirror surfaces(PNNPNP). The '079 patent teaches that both PPNPNP and PNNPNP reimagingconfigurations are advantageous with a physically accessibleintermediate image located between the fourth and fifth mirror. In amanner similar to the '240 and '310 patents, the '079 patent teaches theuse of an aperture stop at the secondary mirror and a chief ray thatdiverges from the optical axis after the secondary mirror.

The '079 patent teaches that the use of a convex tertiary mirror enablesa large reduction in low-order astigmatism. This particular arrangementof optical power is advantageous for achieving a high level ofaberration correction without using high incidence angles or extremelylarge aspheric departures. For both embodiments, all aspheric departuresare below 15 μm and most are below 10 μm. Like the '240 patent, the '079patent makes a significant teaching related to EUV via the use of lowincidence angles on each of the reflective surfaces. The PPNPNP andPNNPNP power arrangements promote low incidence angles thus enablingsimple and efficient EUV mirror coatings. The low incidence angles workto minimize coating-induced amplitude variations in the exit pupil,minimize coating-induced phase or optical path difference (OPD)variations in the exit pupil, and generally lower the tolerancesensitivity of the optical system. These factors combine to promoteimproved transmittance and enhanced CD uniformity in the presence ofvariations in focus and exposure.

While the prior art projection optical systems have proven adequate formany applications, they're not without design compromises that may notprovide an optimum solution in all applications. Therefore, there is aneed for a projection optical system that can be used in the extremeultraviolet (EUV) or soft X-ray wavelength region that has a relativelylarge image field with capable of sub 50 nm resolution.

SUMMARY OF THE INVENTION

In view of the above, an EUV optical projection system is providedincluding at least six reflecting surfaces for imaging an object on animage. The system is configured to form an intermediate image along anoptical path from the object to the image between a secondary mirror anda tertiary mirror, such that a primary mirror and the secondary mirrorform a first optical group and the tertiary mirror and a fourth mirror,a fifth mirror and a sixth mirror form a second optical group. Thesecondary mirror is concave, and the tertiary mirror is convex.

The system may further include an aperture stop located along theoptical path from the object to the image between the primary mirror andthe secondary mirror. This aperture stop may be disposed off each of thefirst mirror and the second mirror.

The system may be further configured such that a chief ray from acentral field point converges toward or propagates approximatelyparallel to the optical axis while propagating between the secondarymirror and the tertiary mirror. The primary mirror may be physicallylocated closer to the object than the tertiary mirror.

The system may be further configured such that a chief ray from acentral field point diverges away from the optical axis whilepropagating between the secondary mirror and the tertiary mirror. Thetertiary mirror may be physically located closer to the object than theprimary mirror.

The primary mirror is preferably concave, the fourth mirror ispreferably concave, the fifth mirror is preferably convex and the sixthmirror is preferably concave.

The physical distance between the object and the image may besubstantially 1500 mm or less, and may further be substantially 1200 mmor less.

The system preferably has a numerical aperture at the image greater than0.18.

Each of the six reflecting surfaces preferably receives a chief ray froma central field point at an incidence angle of less than substantially15°, preferably less than substantially 15°, and five of the sixreflecting surfaces preferably receives a chief ray from a central fieldpoint at an incidence angle of less than substantially 11°, preferablyless than substantially 9°.

The system is preferably configured to have a RMS wavefront error of0.017× or less, and may be between 0.017λ, and 0.011λ.

In another embodiment, the shortcomings of the prior art are overcome bya projection objective having an object plane and an image plane and alight path for a bundle of light rays from the object plane to the imageplane. The six mirrors of the objective are arranged in the light pathfrom the object plane to the image plane. In such an embodiment themirror closest to the image plane where e.g. an object to be illuminatedsuch as a wafer is situated is arranged in such a way that an image-sidenumerical aperture is NA≧0.15. In this application the image-sidenumerical aperture is understood to be the numerical aperture of thebundle of light rays impinging onto the image plane. Furthermore, themirror arranged closest to the image plane of the objective is arrangedin such a way that the image-side free working distance corresponds atleast to the used diameter of the mirror next to the wafer. In apreferred embodiment the image-side free working distance is at leastthe sum of one-third of the used diameter of the mirror next to theimage plane and a length between 20 and 30 mm. In an alternativeembodiment the image-side free working distance is at least 50 mm. In aparticularly preferred embodiment, the image-side free working distanceis 60 mm. In this application the free working distance is defined asthe distance of the vertex of the surface of the mirror next to theimage plane and the image plane. All surfaces of the six mirrors in thisapplication are rotational-symmetric about a principal axis (PA). Thevertex of a surface of a mirror is the intersection point of the surfaceof a mirror with the principal axis (PA). Each mirror has a mirrorsurface. The mirror surface is the physical mirror surface upon whichthe bundle of light rays traveling through the objective from the objectplane to the image plane impinge. The physical mirror surface or theused area of a mirror can be an off-axis or an on-axis mirror segmentrelative to the principal axis (PA).

In another embodiment, a projection objective that comprises six mirrorsis characterized by an image-side numerical aperture, NA, greater than0.15 and an arc-shaped field width, W, at the wafer in the range 1.0mm≦W. The peak-to-valley deviation, A, of the aspheres are limited withrespect to the best fitting sphere of the physical mirror surface of allmirrors by:A≦19 μm−102 μm (0.25−NA)−0.7 μm/mm (2 mm−W).

In a preferred embodiment, the peak-to-valley distance A of the aspheresis limited with respect to the best fitting sphere of the off-axissegments of all mirrors by:A≦12 μm−64 μm (0.25−NA)−0.3 μm/mm (2 mm−W).

According to yet another embodiment, a projection objective thatincludes six mirrors is characterized by an image-side numericalaperture NA≧0.15 and an image-side width of the arc-shaped field W≧1 mm,and the angles of incidence AOI are limited for all rays of the lightbundle impinging a physical mirror surface on all six mirrors S1, S2,S3, S4, S5, S6 by:AOI≦23°−35°(0.25−NA)−0.2°/mm (2 mm−W).wherein the angles of incidence AOI refer to the angle between theincident ray and the normal to the physical mirror surface at the pointof incidence. The largest angle of any incident bundle of light raysoccurring on any of the mirrors is always given by the angle of abundle-limiting ray.

Preferably, an embodiment of the invention would encompass all three ofthe above aspects, e.g., an embodiment in which the free optical workingdistance would be more than 50 mm at NA=0.20 and the peak-to-valleydeviation of the aspheres, as well as the angles of incidence, would liein the regions defined above.

The asphericities herein refer to the peak-to-valley (PV) deviation, A,of the aspherical surfaces with respect to the best fitting sphere ofthe physical mirror surface of an specific mirror. The physical mirrorsurface of a specific mirror is also denoted as the used area of thisspecific mirror. The aspherical surfaces are approximated in theexamples by using a sphere. The sphere has a center on the figure axisvertex of the mirror. The sphere intersects the asphere in the upper andlower endpoint of the used area in the meridian section. The dataregarding the angles of incidence always refer to the angle between theincident ray and the normal to the physical mirror surface at the pointof incidence. The largest angle of any incident bundle of light raysoccurring on any of the physical mirror surfaces is always given by theangle of a bundle-limiting ray. The used diameter or the diameter of thephysical mirror surface will be defined here and below as the envelopecircle diameter of the physical mirror surface or the used area of amirror, which is generally not circular.

In a preferred embodiment the free working distance is 60 mm.

The objective can be used not only in the EUV, but also at otherwavelengths, without deviating from the scope of the invention. In anyrespect, however, to avoid degradation of image quality, especiallydegradation due to central shading, the mirrors of the projectionobjectives should be arranged so that the light path of the bundle oflight rays traveling from the object plane to the image plane isobscuration-free. Furthermore, to provide easy mounting and adjusting ofthe system, the physical mirror surfaces have a rotational symmetry to aprincipal axis (PA). Moreover, to have a compact design with anaccessible aperture and to establish an obscuration-free light path ofthe bundle of light rays traveling from the object plane to the imageplane, the projection objective device is designed in such a way that anintermediate image of the object situated in the object plane is formedafter the fourth mirror. In such systems, it is possible that theaperture stop is situated in the front, low-aperture objective part,with a pupil plane conjugated to the aperture stop imaged in the focalplane of the last mirror. Such a system ensures telecentricity in theimage plane.

In an preferred embodiment of the invention, the aperture stop is freelyaccessible and arranged in the light path from the object plane to theimage plane between the second and third mirror. Good accessibility ofthe aperture stop is ensured when the ratio of the distance between thefirst and third mirror to the distance between the first and secondmirror lies in the range of:0.5<S1S3/S1S2<2.As defined for the free working distance in general a distance betweentwo mirrors is the distance of the vertices of the surfaces of thesemirrors.

Furthermore, in order to prevent vignetting of the light running fromthe third to the fourth mirror, by the aperture stop arranged betweenthe second and third mirror, the ratio of the distance between thesecond mirror and aperture stop to the distance between the third mirrorand the aperture stop lies in the range:0.5<S2 aperture/(S3 aperture)<2.

In such a system, the angles of incidence on the physical mirrorsurfaces in the front part of the objective are reduced.

An aperture stop which physically lies between the second mirror, S2,and the first mirror, S1, must be formed at least partially as a narrowring in order to avoid clipping of light moving from S1 to S2. In such adesign, there is a danger that undesirable direct light or lightreflected on S1 and S2, will pass outside the aperture ring and reachthe image plane and thus the wafer. However, if the aperture stop isplaced in the light path between the second and third mirror andphysically close to the first mirror (which can be easily achievedmechanically), an efficient masking of this undesired light is possible.The aperture stop can be designed both as an opening in the first mirroror an opening which is arranged behind the first mirror.

In another embodiment of the invention, the aperture stop is arranged onor near the second mirror. Arrangement of the aperture on a mirror hasthe advantage that it is easier to manufacture.

In order to ensure an obscuration-free ray path with simultaneously lowangles of incidence, the ratio of the distance between the first andthird mirrors (S1S3) to the distance between the first and secondmirrors (S1S2) lies in the range:0.3≦S1S3/S1S2≦2.0,while the ratio of the distance between the second and third mirrors(S2S3) to the distance between the third and fourth mirrors (S3S4) liesin the range:0.7≦S2S3/S3S4≦1.4.

In order to be able to make the necessary corrections of imaging errorsin the six-mirror systems, in a preferred embodiment, all six mirrorsare designed to be aspherical. However, an alternative embodimentwhereby at most five mirrors are aspherical can simplify themanufacturing, because it is then possible to design one mirror,preferably the largest mirror, i.e., the quaternary mirror, in the formof a spherical mirror. Moreover, it is preferred that the second tosixth mirror be in a concave-convex-concave-convex-concave sequence.

In order to achieve a resolution of at least 50 nm, the design part ofthe rms wavefront section of the system should be at most 0.07λ andpreferably 0.03λ:

Advantageously, in the embodiments of the invention, the objectives arealways telecentric on the image-side.

In projection systems which are operated with a reflection mask, atelecentric light path on the object-side is not possible withoutillumination through a beam splitter which reduces the transmissionstrongly. One such device is known from JP 95 28 31 16.

In systems with transmission mask, the projection objective can betelecentric on the object side. In these embodiments, the first mirroris preferably concave.

The telecentericity error in the image plane, where the wafer issituated should not exceed 10 mrad and is typically between 5 mrad and 2mrad, with 2 mrad being preferred. This ensures that changes of theimaging ratio remain within tolerable limits over the depth of focus.

In an preferred embodiments of the invention, the six mirror objectivecould comprise a field mirror, a reducing three-mirror subsystem and atwo-mirror subsystem.

In addition to the projection objective also a projection exposureapparatus is shown, that includes at least a projection objectivedevice. In a first embodiment, the projection exposure apparatus has areflection mask, while in an alternative embodiment, it has atransmission mask. Preferably, the projection exposure apparatusincludes an illumination device for illuminating an off-axis arc-shapedfield and the system is designed as an arc-shaped field scanner.Furthermore, the secant length of the scan slit is at least 26 mm andthe ring width is greater than 0.5 mm.

The invention will be described below with the aid of the drawings asexamples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a plan view of an EUV optical projection system accordingto a first preferred embodiment.

FIG. 2 schematically illustrates the geometry of the arcuate ring fieldaccording to the preferred embodiments at the object.

FIG. 3 shows a plan view of an EUV optical projection system accordingto a second preferred embodiment.

FIG. 4 shows a plan view of an EUV optical projection system accordingto a third preferred embodiment.

FIG. 5 shows the ring field in the object plane of the objective.

FIG. 6 shows an embodiment with an intermediate image, a freelyaccessible aperture stop between a second and third mirror, and a imageside numerical aperture of 0.2.

FIG. 7 shows a prior art six-mirror objective arrangement forwavelengths >100 nm as disclosed in U.S. Pat. No. 5,686,728.

FIG. 8 shows a second embodiment with an aperture stop between thesecond and third mirror at the first mirror.

FIG. 9 shows a third embodiment with an aperture stop on the secondmirror and a working distance of 59 mm.

FIG. 10 shows a fourth embodiment with an intermediate image, a imageside numerical aperture NA of 0.28 as well as a free working distance onthe image-side which is at least the sum of one-third of the usefuldiameter of the mirror nearest to the wafer and a length which liesbetween 20 and 30 mm.

FIG. 11 shows a fifth embodiment of a system with an intermediate imageand a image side numerical aperture NA of 0.30.

FIGS. 12A and 12B show the used diameter for different physical mirrorsurfaces or used areas of a mirror.

INCORPORATION BY REFERENCE

What follows is a cite list of references which, in addition to thatwhich is described in the background and brief summary of the inventionabove, are hereby incorporated by reference into the detaileddescription of the preferred embodiments, as disclosing alternativeembodiments of elements or features of the preferred embodiment nototherwise set forth in detail below. A single one or a combination oftwo or more of these references may be consulted to obtain a variationof the preferred embodiments described below. Further patent, patentapplication and non-patent references, and discussion thereof, cited inthe background and/or elsewhere herein are also incorporated byreference into the detailed description of the preferred embodimentswith the same effect as just described with respect to the followingreferences:

U.S. Pat. Nos. 5,063,586, 5,071,240, 5,078,502, 5,153,898, 5,212,588,5,220,590, 5,315,629, 5,353,322, 5,410,434, 5,686,728, 5,805,365,5,815,310, 5,956,192, 5,973,826, 6,033,079, 6,014,252, 6,188,513,6,183,095, 6,072,852, 6,142,641, 6,226,346, 6,255,661 and 6,262,836;

European patent applications no. 0 816 892 A1 and 0 779 528 A; and

“Design of Reflective Relay for Soft X-Ray Lithography”, J. M. Rodgers,T. E. Jewell, International Lens Design Conference, 1990;

“Reflective Systems design Study for Soft X-ray Projection Lithography”,T. E. Jewell, J. M. Rodgers, and K. P. Thompson, J. Vac. Sci. Technol.,November/December 1990.

“Optical System Design Issues in Development of Projection Camera forEUV Lithography”, T. E. Jewell, SPIE Volume 2437, pages 340-347;

“Ring-Field EUVL Camera with Large Etendu”, W. C. Sweatt, OSA TOPS onExtreme Ultraviolet Lithography, 1996; and

“Phase Shifting Diffraction Interferometry for Measuring ExtremeUltraviolet Optics”, G. E. Sommargaren, OSA TOPS on Extreme UltravioletLithography, 1996;

“EUV Optical Design for a 100 nm CD Imaging System”, D. W. Sweeney, R.Hudyma, H. N. Chapman, and D. Shafer, SPIE Volume 3331, pages 2-10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Three specific preferred embodiments relating to this optical projectionsystem are described.

First Preferred Embodiment

FIG. 1 shows a plan view of a first preferred embodiment, and, taking inconjunction with Table 1 and Table 2, provides an illustrative,exemplary description of this embodiment. Light impinges on an object,e.g. a reflective mask or reticle from an illumination system and isdirected to concave mirror M1 after which it reflects from the mirror M1and passes through a physically accessible aperture stop APE that islocated between Mirror M1 and M2. This aperture stop APE is located asubstantial distance from the first concave mirror M1 and, likewise,this aperture stop APE is located a substantial distance from concavemirror M2. After the illumination reflects off concave mirror M2, thelight comes to a focus at an intermediate image IMI that is located inclose proximity to convex mirror M3. From mirror M3 the illumination isdirected toward concave mirror M4 where the light is nearly collimatedand directed toward convex mirror M5. Upon reflection from mirror M5,the light impinges on concave mirror M6 where it is reflected in atelecentric manner (the chief rays are parallel to the optical axis OA)and focused on the image IM. A semiconductor wafer is typically arrangedat the position of the image IM. Since a concave optical surface haspositive optical power (P) and a convex optical surface has negativeoptical power (N), this present embodiment may be characterized as aPPNPNP configuration.

Although there are many ways to characterize this optical system, oneconvenient way is to break the system into two groups G1 and G2.Starting at the object OB, the first group G1 is comprised the concavemirror pair M1 and M2. This group forms an intermediate image IMI at amagnification of about −0.8× between mirror M2 and mirror M3. Theremaining four mirrors (convex mirror M3, concave mirror M4, convexmirror M5 and concave mirror M6) comprise the second imaging or relaygroup G2. This second group G2 works at a magnification of approximately−0.3×, resulting in 4× reduction (the reduction ratio is the inverse ofthe absolute value of the optical magnification) of the object OB at theimage IM.

The optical prescription of the first embodiment of FIG. 1 is listed inTable 1 and Table 2. The aspheric mirror surfaces are labeled A(1)-A(6)in the tables with A(1) corresponding to mirror M1, A(2) correspondingto mirror M2, and so on. Four additional surfaces complete thedescription of this illustrative and exemplary embodiment with object OBand image IM representing the planes, where in a lithographic apparatusthe mask and the wafer are arranged. A surface designation is also madefor the location of the aperture stop APE and intermediate image IMI.After each surface designation, there are two additional entries listingthe vertex radius of curvature (R) and the vertex spacing between theoptical surfaces. In this particular embodiment, each of the surfaces isrotationally symmetric conic surface with higher-order polynomialdeformations. The aspheric profile is uniquely determined by its K, A,B, C, D, and E values. Each mirror uses 4th, 6th, 8th, 10th, and 12thorder polynomial deformations. The sag of the aspheric surface (through12th order) in the direction of the z-axis (z) is given by:$z = {\frac{{ch}^{2}}{1 + \sqrt{1 - {\left( {1 + k} \right)c^{2}h^{2}}}} + {A\quad h^{4}} + {Bh}^{6} + {Ch}^{8} + {Dh}^{10} + {Eh}^{12}}$where h is the radial coordinate; c is the vertex curvature of thesurface (1/R); and A, B, C, D, and E are the 4th, 6th, 8th, 10th, and12th order deformation coefficients, respectively. These coefficientsare listed in Table 2.

The optical system of this first preferred embodiment is designed toproject a ring field format that is illuminated with extremelyultraviolet (EUV) or soft X-ray radiation. The numerical aperture NAO atthe object OB is 0.050 radians; at a 4× reduction this corresponds to anumerical aperture NA of 0.20 at the image IM. The ring field 21 at theobject OB is shown with FIG. 2. It is centered at 118 mm from theoptical axis, which contains the vertex of each of the aspheric mirrors.This annular field extends from 114 mm to 122 mm forming an arcuate slitwith a width 23 of 8 mm. The extent 25 of the ring field 21perpendicular to the scan direction 27 becomes 104 mm. The central fieldpoint is denoted with the reference sign 29. At 4× reduction, this ringfield becomes 2.0 mm wide in the scan direction at the image.

As a result of the distribution of optical power and location of theaperture stop APE, the incidence angles are well controlled so that thedesign is compatible with EUV or soft X-ray multilayer coatings. Asmeasured by the chief ray CR from the central field point 29, thissystem exhibits very low incidence angles ranging from 2.9° to 12.5°.The chief ray incidence angles for the chief ray CR from the centralfield point 29 are: Object: 5.2°; M1: 6.5°; M2: 5.0°; M3: 12.5°; M4:5.6°, M5: 8.6°, and M6: 2.9°. These low incidence angles are a keyenabling element for EUV lithography since (1) they minimize themultilayer induced amplitude and phase errors that have an adverseimpact to lithographic performance and (2) enable simplified coatingdesigns that do not rely heavily on the use of laterally graded coatingprofiles. With poor design (i.e., failure to minimize these incidenceangles), these multilayer-induced amplitude and phase errors can lead tocritical dimension (CD) errors that are easily greater than 20% of thenominal linewidth, making the system unusable for productionapplications.

Besides the low incidence angles, a preferred system further enables EUVlithography by utilizing mirrors with low peak aspheric departure. Themaximum peak departure, contained on mirror M1, is 25.0 μm. The othermirrors have low-risk aspheres with departures that range from 0.5 μm to14 μm. The low aspheric departures of the mirror surfaces facilitatevisible light metrology testing without a null lens or ComputerGenerated Hologram CGH, resulting in surface figure testing to a highdegree of accuracy. An aspheric mirror with a very large peak departureis unproducible because it cannot be measured to the required accuracyto realize lithographic performance.

Table 3 summarizes the performance of the PPNPNP configuration ofFIG. 1. The table demonstrates that this first preferred embodiment isable to achieve lithographic performance with a resolution on the orderof 30 nm (assuming a k1-factor of approximately 0.5). The location ofthe aperture stop APE is selected so that the third order astigmatismcontribution from the strong concave secondary mirror M2 is made verysmall. The strongly undercorrected astigmatic contribution from theprimary mirror M1 comes from the aspheric departure on M1 and isbalanced by the M3/M4 combination. Considering the system without anyaspheres, the location of the aperture stop APE also effectivelybalances the third-order coma and distortion contributions from theprimary mirror M1 and secondary mirror M2. A hyperbolic profile is addedto the primary mirror M1 in such a way as to create a largeundercorrected spherical contribution, coma contribution, andastigmatism contribution, thus promoting good aberration correctionallowing the residual wavefront error (departure from the idealreference sphere) to remain exceedingly small. In fact, aberrationcorrection and resulting aberration balance reduces the composite RMSwavefront error is only 0.0125λ (0.17 nm), with simultaneous correctionof the static distortion to less than 2 nm across the field.

This optical projection system has further benefits in that the systemof FIG. 1 may be scaled in either numerical aperture or field. Forexample, it is desirable to scale this concept to larger numericalaperture to improve the modulation in the aerial image thus allowing 30nm resolution with a less aggressive k1-factor. The results of a simplescaling experiment demonstrate that this preferred embodiment easilysupports such scaling to larger numerical apertures. Without making anymodifications, an analysis of the composite root mean square (RMS)wavefront error was made at a numerical aperture of 0.24, whichrepresents a 20% increase to the value shown in Table 2. The compositeRMS wavefront error was found to be 0.0287% (0.38 nm), a level thatsupports lithographic quality imaging.

Referring to FIG. 2, it is desirable to increase the field of view inthe scan direction to increase the number of wafers per hour (WPH) thatthe lithographic apparatus can process. The idea is that more area canbe printed per unit time with a wider arcuate slit. The results ofanother simple scaling experiment demonstrate that this preferredembodiment easily supports increases in field width. Without making anymodifications, an analysis of the composite RMS wavefront error was madeover a 3 mm wide arcuate slit, which represents a 50% increase to thevalue shown in Table 2. The composite RMS wavefront error was found tobe 0.0285λ (0.38 nm), again a level that supports lithographic qualityimaging.

Second Preferred Embodiment

In a second of these general embodiments, an optical projection systemfor extreme ultraviolet (EUV) lithography including six mirrors arrangedin a PPNPNP configuration is disclosed. The plan view of this secondpreferred embodiment is shown in FIG. 3, which demonstrates a PPNPNPconfiguration designed for EUV lithography at a wavelength of 13.4 nm.Like the first preferred embodiment, the system is reimaging, and unlikethe '310 and '079 embodiments, locates the intermediate image IMI′before the second mirror pair. In this example, the intermediate imageIMI′ is located between mirror M2′ and M3′, helping to promote lowincidence angle variation across mirror M5′. This construction alsoenables low mean incidence angles on mirror M1′, M2′, M4′, and M6′.These low incidence angles are advantageous for maintaining goodmultilayer compatibility. The aperture stop APE′ is located between M1′and M2′ and is significantly spaced from either mirror, e.g., more than200 mm.

In addition to the features outlined by the first preferred embodiment,this second preferred embodiment teaches that the tertiary mirror M3′may be located on the object side of the primary mirror M1′ (i.e.,closer to the object OB′ than the primary mirror M1′). This featuredeparts drastically from the teaches of the prior art that show thetertiary mirror must be located either in close proximity to the primarymirror ('079 patent) or on the image side of the primary mirror ('310patent). This location of mirror M3′ enables a reduction in the overalllength from object plane OB to image plane IM (total track length) bysome 250 mm. This decrease in total track length is accomplished byshifting the tertiary mirror from the image side of the primary mirrorM1′ to the object side of the primary mirror M1′ and then decreasing thedistance between mirror M1′ and mirror M6′. This also allows the parentdiameter of the tertiary mirror M3′ to be smaller than either theprimary mirror M1′ or the secondary mirror M2′. These changes affect theangular condition of the chief rays upon reflection from the secondarymirror M2′. Prior art teaches that the chief ray from the central fieldpoint must diverge from the optical axis after reflection from thesecondary mirror ('310 patent, '079 patent, etc.), but now the chief rayCR′ assumes a more parallel condition with respect to the optical axisOA′. In this second embodiment, this chief ray CR′ is made identicallyparallel to the optical axis OA′. This change in chief ray angle impactsthe aberration balance in the design enough to form a distinct localminima, so that the residual aberration set seen in a Zemikedecomposition of the wavefront differs from that of the first preferredembodiment.

The optical prescription of this second preferred embodiment of FIG. 3is listed in Table 4 and Table 5. The aspheric mirror surfaces arelabeled A(1)-A(6) in the tables with A(1) corresponding to mirror M1,A(2) corresponding to mirror M2, and so on.

Like the first preferred embodiment, the object OB′ will be projected tothe image IM′ at 4× reduction in a ring field format with a telecentricimaging bundle (chief rays parallel to the optical axis OA′ at the imageIM′). Table 6 provides a performance summary demonstrating that thispreferred embodiment is capable of lithographic performance at awavelength of 13.4 nm. For comparison to the first embodiment, thissecond preferred embodiment also utilizes a numerical aperture NA of0.20 at the image IM′ and projects a 2 mm wide field in the scandirection. The system is compatible with reflective multilayer coatingssince the incidence angles at each mirror are relatively small. Asmeasured by the chief ray CR′ from the central field point 29′, theincidence angles range from 3.9° to 14.60. The exact chief ray incidenceangles for the chief ray CR′ from the central field point 29′ are:Object OB′: 5.6°; M1: 7.2°; M2: 4.4°; M3: 14.6°; M4: 8.8°, M5: 9.7°, andM6: 3.9°. Again, these low incidence angles are a key enabling elementfor EUV lithography since the low incidence angles minimize themultiplayer induced amplitude and phase errors that have an adverseimpact to lithographic performance.

The composite RMS wavefront error across the field is 0.0131λ (0.18 nm),ranging from 0.0095λ (0.13 nm) at the best field point to 0.0157λ (0.21nm) at the worst. The distortion of the chief ray has been reduced toless than 1 nm across the field. Clearly this combination of telecentricimaging, a highly corrected wavefront, and essentially no distortiondemonstrates that this system is suitable for modern lithography at softx-ray or extreme ultratviolet wavelengths.

This preferred embodiment has further advantages in that the system ofFIG. 3 may be scaled in either numerical aperture or field to addresseven more advanced requirements. The results of a simple numericalaperture scaling experiment demonstrate that this preferred embodimenteasily supports scaling to larger numerical apertures. Without makingany modifications, an analysis of the composite root mean square (RMS)wavefront error was made at a numerical aperture of 0.22, whichrepresents a 10% increase to the value shown in Table 4. The compositeRMS wavefront error was found to be 0.027λ (0.36 nm), a level thatsupports lithographic quality imaging.

The results of another simple scaling experiment demonstrate that thispreferred embodiment easily supports increases in field width. Withoutmaking any modifications, an analysis of the composite RMS wavefronterror was made over a 3 mm wide arcuate slit, which represents a 50%increase to the value shown in Table 6. The composite RMS wavefronterror was found to be 0.028% (0.38 nm), again a level that supportslithographic quality imaging.

Third Preferred Embodiment

The third preferred embodiment is shown in FIG. 4. Like the first andsecond preferred embodiments, this system utilizes a re-imaging PPNPNPconfiguration with a physically accessible aperture stop APE″ that islocated between the primary mirror M1″ and secondary mirror M2″. Andlike the first and second embodiments, the intermediate image IMI″ islocated between the secondary mirror M2″ and the tertiary mirror M3″.Similar to the second embodiment, the tertiary mirror M3″ is located onthe object side of the primary mirror M1″. This particular embodimentdiffers from the second preferred embodiment in that the chief ray CR″from the central field point 29″ converges toward the optical axis OA″after reflection from the secondary mirror M2″, thus forming anotheradvantageous projection system with distinct characteristics.

The optical prescription for this third embodiment of FIG. 4 is listedin Table. 7 and Table 7. Table 7 lists the vertex radius of curvature aswell as the separation between these mirrors along the optical axis.Each mirror is aspheric and labeled A(1)-A(6) in the tables with A(1)corresponding to mirror M1″, A(2) corresponding to mirror M2″, and soon. The prescription of the aspheric surface deformation per equation(1) is listed in Table 8. Taken together with the information providedin Table 9, an illustrative and exemplary description of this preferredembodiment is disclosed. Like the first two preferred embodiments, theobject OB″, e.g. a pattern on mask or reticle, will be projected to theimage IM″ at 4× reduction in a ring field format with a telecentricimaging bundle (chief rays parallel to the optical axis at the image).At the image″ typically a semiconductor wafer is arranged. Table 6provides a performance summary demonstrating that this preferredembodiment is capable of lithographic performance at a wavelength of13.4 nm. For comparison purposes, this third preferred embodiment alsoutilizes a numerical aperture NA of 0.20 at the image IM″ and projects a2 mm wide field in the scan direction. The system is compatible withreflective multilayer coatings since the incidence angles at each mirrorare relatively small. As measured by the chief ray CR″ from the centralfield point 29″, the incidence angles range from 3.90 to 13.9°. Theexact chief ray incidence angles from the central field point are:Object OB″: 6.6°; M1: 8.0°; M2: 4.4°; M3: 13.9°; M4: 8.6°, M5: 9.6°, andM6: 3.9°. Again, these low incidence angles are a key enabling elementfor EUV lithography since the low incidence angles minimize themultiplayer induced amplitude and phase errors that have an adverseimpact to lithographic performance.

The composite wavefront error across the field is 0.0203λ (0.27 nm),ranging from 0.0148λ (0.20 nm) at the best field point to 0.0243λ (0.33nm) at the worst. The distortion of the chief ray has been reduced toless than 1 nm across the field. Clearly this combination of telecentricimaging, a highly corrected wavefront, and essentially no distortiondemonstrates that this system is suitable for modern lithography at softx-ray or extreme ultratviolet wavelengths. The design can also be scaledin numerical aperture or field like second preferred embodiment.

The optical design descriptions provided above for the first-thirdembodiments herein demonstrate an advantageous catoptric projectionsystem concept for EUV lithography. While these embodiments have beenparticularly described for use in a 13.4 nm tool, the basic concept isnot limited to use with lithographic exposure tools at this wavelength,either shorter or longer, providing a suitable coating material existsin the soft x-ray region of the electromagnetic spectrum.

While exemplary drawings and specific embodiments of the presentinvention have been described and illustrated, it is to be understoodthat that the scope of the present invention is not to be limited to theparticular embodiments discussed. Thus, the embodiments shall beregarded as illustrative rather than restrictive, and it should beunderstood that variations may be made in those embodiments by workersskilled in the arts without departing from the scope of the presentinvention as set forth in the claims that follow, and equivalentsthereof. For example, one skilled in the art may reconfigure theembodiments described herein to expand the field of view, increase thenumerical aperture, or both, to achieved improvements in resolution orthroughput. TABLE 1 Optical prescription first preferred embodimentVertex radius of Element number curvature Thickness (mm) Glass Object OBINFINITY 806.775 A(1) −1997.63 −328.184 REFL Aperture Stop INFINITY−399.404 APE A(2) 1148.069 649.7918 REFL Intermediate INFINITY 132.9323image IMI A(3) 486.7841 −277.569 REFL A(4) 660.9159 890.6587 REFL A(5)393.8628 −476.472 REFL A(6) 580.3377 501.472 REFL Image IM

TABLE 2 Aspheric prescription Aspheric K A B C D E A(1) −9.1388E+015.4676E−10 7.0301E−15 −1.4409E−19 2.1657E−25 5.5712E−3

A(2) −6.4930E−01 3.7924E−11 3.2952E−18 −1.1462E−21 8.4115E−26 −4.9020E−3

  A(3) −2.3288E−01 3.3571E−10 1.8240E−14 −1.9218E−19 −4.2667E−23 2.9468E−2

A(4) −6.4180E−03 3.9345E−11 1.8257E−16 −6.9023E−22 1.3692E−26 −6.2042E−3

  A(5)  1.5857E+00 −1.7764E−09  7.7970E−14 −1.2619E−18 5.4017E−22−3.8012E−2

  A(6)  8.98S4E−02 −4.2455E−12  1.4898E−17  1.4824E−22 −7.0550E−28 6.6775E−3

TABLE 3 Performance summary first preferred embodiment MetricPerformance Wavelength 13.4 nm Numerical aperture (image) 0.20 Ringfieldformat (image) i. Radius 30.0 mm ii. Width 2.0 mm iii. Chord 26.0 mmReduction ratio (nominal) 4:1 Overall length (mm) 1500 mm RMS wavefronterror (waves @ λ = 13.4 nm) i. Composite 0.0125λ ii. Variation0.0076λ-0.0167λ Chief ray distortion (max) 1.9 nm Exit pupil locationInfinity Max. aspheric departure across instantaneous clear aperture(ICA) i. M1 25.0 μm ii. M2 0.5 μm iii. M3 1.4 μm iv. M4 14.0 μm v. M53.0 μm vi. M6 3.8 μm

TABLE 4 Optical prescription second preferred embodiment Vertex radiusof Element number curvature Thickness (mm) Glass Object Plane OB′INFINITY 786.7828 A(1) −1522.9647 −275.3849 REFL Aperture Stop INFINITY−461.3979 APE′ A(2) 922.8035 452.3057 REFL Intermediate INFINITY 95.0000image IMI′ A(3) 273.0204 −218.5016 REFL A(4) 511.1320 834.1959 REFL A(5)434.1472 −326.2172 REFL A(6) 440.9571 363.2172 REFL Image IM′

TABLE 5 Aspheric prescription second preferred embodiment Aspheric K A BC D E A(1) −6.5661E−04  3.6028E+01 2.7656E−09 1.3237E−14 5.6475E−201.4711E−2

A(2) 1.0837E−03 −3.0142E+00  3.2384E−10 −6.8499E−16  −1.8748E−20 1.0985E−2

A(3) 3.6627E−03 1.9328E+00 −1.6611E−08  −4.9082E−13  2.9169E−17−3.8673E−2

  A(4) 1.9564E−03 −1.2442E−01  −1.0927E−11  2.7712E−16 −2.0608E−21 3.6395E−2

A(5) 2.3034E−03 8.5377E+00 −6.9001E−09  −2.2929E−13  −8.9645E−18 −2.1791E−2

  A(6) 2.2678E−03 1.4526E−01 3.2069E−11 3.3003E−16 5.1329E−21 −1.7296E−2

 

TABLE 6 Performance summary second preferred embodiment MetricPerformance Wavelength 13.4 nm Numerical aperture (image) 0.20 Ringfieldformat (image) i. Radius 30.0 mm ii. Width 2.0 mm iii. Chord 26.0 mmReduction ratio (nominal) 4:1 Overall length (mm) 1250 RMS wavefronterror (waves @ λ = 13.4 nm) i. Composite 0.0131λ ii. Variation0.0095λ-0.0157λ Chief ray distortion (max) 0.9 nm Exit pupil locationInfinity Max. aspheric departure across instantaneous clear aperture(ICA) i. M1′ 18.0 μm ii. M2′ 6.2 μm iii. M3′ 8.7 μm iv. M4′ 28.0 μm v.M5′ 7.0 μm vi. M6′ 7.0 μm

TABLE 7 Optical prescription third preferred embodiment Vertex radius ofElement number curvature Thickness (mm) Glass Object OB″ INFINITY708.2375 A(1) −1351.9353 −222.3328 REFL Aperture Stop APE″ INFINITY−435.9047 A(2) 801.1198 389.5537 REFL Intermediate image INFINITY85.9324 IMI″ A(3) 257.6903 −223.6826 REFL A(4) 508.9915 827.9429 REFLA(5) 434.7744 −321.5090 REFL A(6) 436.7586 358.5090 REFL Image IM″

TABLE 8 Aspheric prescription third preferred embodiment Aspheric K A BC D E A(1) −7.3968E−04  1.8042E+00 2.2388E−09  4.0136E−15 6.8479E−19−1.2865E−2

A(2) 1.2483E−03 −2.6267E+00  4.4819E−10 −1.7571E−15 5.8143E−20−3.7874E−2

A(3) 3.8806E−03 −8.5604E−01  2.2165E−08 −6.7204E−12 1.1406E−15−1.0131E−1

A(4) 1.9647E−03 −7.7387E−02  −3.8053E−11  −1.2483E−15 2.8880E−20−3.4746E−2

A(5) 2.3000E−03 8.3687E+00 −6.1944E−09  −1.9683E−13 −1.6280E−17  4.8296E−2

A(6) 2.2896E−03 1.3269E−01 5.6594E−11  5.5533E−16 −1.1978E−21  7.3097E−2

TABLE 9 Performance summary third preferred embodiment MetricPerformance Wavelength 13.4 nm Numerical aperture (image IM″) 0.20Ringfield format (image IM″) i. Radius 30.0 mm ii. Width 2.0 mm iii.Chord 26.0 mm Overall length (mm) 1156 Reduction ratio (nominal) 4:1 RMSwavefront error (waves @ λ = 13.4 nm) i. Composite 0.0203λ ii. Range0.0148λ-0.0243λ Chief ray distortion (max) 1.5 nm Exit pupil locationInfinity Max. aspheric departure across instantaneous clear aperture(ICA) i. M1″ 17.3 μm ii. M2″ 6.4 μm iii. M3″ 9.7 μm iv. M4″ 32.2 μm v.M5″ 6.7 μm vi. M6″ 6.7 μm

In FIG. 5 the object field 1100 of a projection exposure apparatus inthe object plane of the projection objective according to the inventionis shown. The object plane is imaged by means of the projectionobjective in an image plane, in which a light sensitive object, forexample a wafer with a light sensitive material is arranged. The imagefield in the image plane has the same shape as the object field. Theobject- or the image field 1100 has the configuration of a segment of aring field. The ring field has an axis of symmetry 1200.

In addition the axis extending the object plane, i.e., the x-axis andthe y-axis are depicted. As can be seen from FIG. 5, the axis ofsymmetry 1200 of the ring field runs in the direction of the y-axis. Atthe same time the y-axis coincides with the scanning direction of anprojection exposure apparatus, which is designed as a ring fieldscanner. The x-direction is thus the direction that stands perpendicularto the scanning direction, within the object plane. The ring field has aso called ring field radius R, which is defined by the distance of thecentral field point 1500 of the image field from the principal axis (PA)of the projection objective. The arc-shaped field in the object plane aswell as in the image plane has a arc shaped field width W, which is theextension of the field in scanning or in y-direction and a secant lengthSL.

In FIGS. 6, 8 and 9, arrangements of the six-mirror projectionobjectives are shown. Each embodiment has a free working distance thatcorresponds at least to the used diameter of the physical mirror surfaceor mirror segment next to the wafer. In contrast, FIG. 7 shows a priorart system for use with wavelengths >100 nm, such as the system of U.S.Pat. No. 5,686,728. In all embodiments shown in FIGS. 6, 8 and 9, thesame reference numbers will be used for the same components and thefollowing nomenclature will be employed:

-   -   first mirror (S1), second mirror (S2), third mirror (S3), fourth        mirror (S4), fifth mirror (S5), and sixth mirror (S6)

In particular, FIG. 6 shows a six-mirror projection objective with a raypath from the object plane 2, i.e. reticle plane to the image plane 4,i.e. wafer plane. The embodiment includes a field mirror S1, which formsa virtual image of an object with an imaging ratio β>0. A three-mirrorsystem formed from S2, S3 and S4 is also provided and produces a real,reduced image of the virtual image as the intermediate image, Z. Lastly,a two-mirror system S5, S6, images the intermediate image Z in the waferplane 4 while maintaining the requirements of telecentricity. Theaberrations of the three-mirror and two-mirror subsystems are balancedagainst one another so that the total system has a high optical qualitysufficient for integrated circuit fabrication applications.

The physical aperture stop B is arranged between the second mirror S2and the third mirror S3. And, as is clear from FIG. 6, the aperture stopis accessible in the ray path between the second mirror S2 and the thirdmirror S3. Furthermore, the distance between the vertex V5 of thesurface of the mirror next to the wafer, i.e., the surface of the fifthmirror S5 in the present embodiment, and the image plane is greater thanthe used diameter of the physical mirror surface of mirror S5. The useddiameter of a physical mirror surface is explained in more detail in thedescription of FIGS. 12A and 12B. In other words, the followingcondition is fulfilled:

physical distance from the vertex V5 of the surface of mirror S5 to theimage plane 4> used diameter of mirror S5.

Other distance requirements are also possible and may be used, such asthe physical distance is (1) greater than the sum of one-third of theused diameter of the mirror next to the wafer, S5, and 20 mm, or (2)greater than 50 mm. In the preferred embodiment, the physical distanceis 60 mm.

Such a physical distance guarantees a sufficiently free working distanceA, and allows the use of optical components compatible for use withwavelengths <100 nm, and preferably wavelengths of 11 to 13 nm. Opticalcomponents in this range include, for example, Mo/Si or Mo/Be multilayersystems, where the typical multilayer systems for λ=13 nm is Mo/Si layerpairs and for λ=11 nm, is Mo/Be systems, both of approximately 70 layerpairs. Reflectivities attainable in such systems are approximately 70%.In the multilayer layer systems, layer stresses of above 350 MPa mayoccur. Stresses of such values may induce surface deformation,especially in the edge regions of the mirror.

The systems according to the invention, as they are shown, for example,in FIG. 5, have:RES=k ₁ λ/NA.

This results in a nominal resolution of at least 50 nm and 35 nm at aminimum numerical aperture of NA=0.2 for k₁=0.77 and %=13 nm, and fork₁=0.64 and λ=11 nm, respectively, where k₁ is a parameter specific forthe lithographic process.

Furthermore, the light path for a bundle of light rays running from theobject plane to the image plane of the objective shown in FIG. 6 isobscuration-free. For example, in order to provide image formats of26×34 mm or 26×52 mm², the projection objectives according to theinvention are preferably used in an arc-shaped field scan projectionexposure apparatus, wherein the secant length of the scan slit is atleast 26 mm.

Numerous masks can be used in the projection exposure apparatus. Themasks or reticle are arranged in the object plane of the projectionobjective. The masks include transmission masks, stencil masks andreflection masks. The projection objective, which is telecentric on theimage side, i.e. in the image plane, can be telecentric ornon-telecentric on the object side, i.e. in the object plane dependingon which mask is used. For example, if the bundle of light rays istelecentric on the object-side when using a reflection mask atransmission-reducing beam splitter must be employed. If the bundle oflight rays is non-telecentric on the object-side, unevennesses of themask leads to dimensional errors in the image. Therefore, the angle ofincidence of the chief ray of the bundle of light rays through thecentral field point 1500 in the object plane is preferably below 10°, sothat the requirements for reticle evenness lies in an achievable range.Moreover, the system of FIG. 6 which is telecentric on the image sidehas an image-side error of telecentry at the wafer level of 1 mrad for aimage side numerical aperture of 0.2.

Due to the high image-side telecentricity, the entrance pupil of thelast mirror S6 is at or near the focal plane of this mirror. Therefore,in systems with an intermediate image as described before, the aperture,B, is in the front, low-aperture objective part preferably in the lightpath between the first and third mirror S1, S3. Thus the pupil planeconjugated with the aperture stop will be imaged in the focal plane ofthe last mirror.

All mirrors S1-S6 of FIG. 6 are designed to be aspherical, with amaximum asphericity of approximately 7.3 μm. The low asphericity of theembodiment shown in FIG. 6 is advantageous from a manufacturing point ofview, since the technological difficulties in processing the surfaces ofthe multilayer mirrors increases proportionally with asphericaldeviation and gradient of the asphere.

The highest angle of incidence of a ray impinging a mirror surface inthe six-mirror objective shown in FIG. 6 occur on the fifth mirror S5and is approximately 18.4°. The maximum variation of the angles ofincidence of the rays within a bundle of light rays impinging onto amirror surface occurs on mirror surface of mirror S5 and isapproximately 14.7°. The wavefront error at λ=13 nm is better than0.032λ; the centroid distortion of the point spread function is <3 mm;and the static, dimension-corrected distortion lies at 4 nm.

A freely accessible aperture stop between the second and third mirror aswell as no vignetting of the bundle of light rays running from S3 to S4by the aperture stop is achieved with small angles of incidence of therays impinging onto the mirror surfaces when the following distanceconditions are fulfilled:0.5<S1S3/S1S2<2and0.5<S2 aperture/(S3 aperture)<2.

Here, the abbreviation S1S3 means the mechanical distance or physicaldistance between the vertices V1 and V3 of the surface of the mirrors S1and S3. And, “S2 aperture” means the mechanical distance between thevertex V2 of the surface of mirror S2 and the aperture. Furthermore, inorder to reduce the angles of incidence on the mirrors in any of theembodiments of FIGS. 6, 8, and 9, the distance from the object plane,where e.g. the reticle is situated to the vertex of the surface of themirror S1 is made smaller than the mechanical distance from the vertexof the surface of mirror S2 to the vertex of the surface of mirror S3,i.e., the following applies:reticle S1<S2S3.

To ensure a sufficient free working distance A not only on the imageside but also on the object side the reticle is situated sufficientlyfar in front of the first mirror next to the object plane, which is inthe present case the surface of the second mirror S2. In the presentcase, for example, the physical distance between the reticle and thevertex V2 of the surface of mirror S2 is 80 mm.

Furthermore, in the embodiments of FIGS. 6 and 8 to 10, the physicaldistance between the mirrors S3 and S6 is chosen that mirrors ofsufficient thickness can be used. Thicker mirrors have sufficientstrength and stability properties that can withstand the high layertensions described above. In these systems, the following relationshipis preferred:0.3 (used diameter S3+used diameter S6)<S3S6.

Here S3S6 denotes the physical distance between the vertex V3 of thesurface of mirror S3 and the vertex V6 of the surface of the mirror S6.

In the following table 10, the parameters of the system represented inFIG. 6 are exemplarily shown in Code V™ nomenclature. The objective is a5× system with a 26×2 mm² arc-shaped field in the image plane, wherein26 mm is the secant length of the arc-shaped field and 2 mm is the widthW of the arc shaped field. Furthermore the numerical aperture is 0.2 onthe image side. The mean image side radius of the system isapproximately 26 mm. TABLE 10 element No. radius Thickness diameter TypeObject INF 80.9127 258.1723 413.0257 S1 A(1) −88.8251 197.5712 REFL−324.2006 195.6194 0.0000 188.6170 S2 A(2) 324.2006 188.7078 REFLaperture 67.1796 423.6214 183.2180 0.0000 S3 A(3) −423.6214 184.7062REFL −74.9270 519.0546 S4 A(4) 498.5484 541.0453 REFL 109.8242 248.6244281.5288 177.5488 S5 A(5) −281.5288 65.0842 REFL S6 A(6) 281.5288187.9549 REFL 78.3999 Image image width 59.9202 53.9889 asphericalconstants: Z = (CURV) Y²/[1 + (1 − (1 + K)(CURV)²Y²)^(1/2)] + (A)Y⁴ +(B)Y⁶ + (C)Y⁸ + (D)Y¹⁰ asphere CURV K A B C D A(1) 0.00031800−27.686599  0.00000E+00 1.32694E−15  2.00546E−20 −8.49471E−25  A(2)0.00094928 −3.998204  0.00000E+00 4.03849E−15 −6.15047E−20 2.73303E−25A(3) 0.00126752 0.424198 0.00000E+00 1.58766E−15 −8.27965E−202.80328E−24 A(4) 0.00123850 0.023155 0.00000E+00 2.46048E−17−1.08266E−22 3.75259E−28 A(5) 0.00329892 2.902916 0.00000E+001.55628E−12 −6.71619E−17 −5.30379E−21  A(6) 0.00277563 0.0729420.00000E+00 2.96285E−16  3.99125E−21 4.55007E−26Reference wavelength = 13 nm

FIG. 7 shows an arrangement of a projection objective formicrolithography with a wavelength of λ<100 nm according to U.S. Pat.No. 5,686,728. Components substantially similar to those of FIG. 6 areprovided with the same reference numbers. As is clear, the physicaldistance between the vertex V5 of the surface of the mirror next to theimage plane S5 and the image plane, where the wafer is situated issignificantly smaller than the used diameter of the fifth mirror S5,lying mainly in the range of approximately 20 mm. This leads to strengthand stability problems for the optics in the EUV region because of theextreme tensions in the layers. Furthermore, the system hasasphericities of +50 μm and a maximum angle of incidence of 38°.

FIG. 8 is an alternative embodiment of a six-mirror system in which theaperture stop is situated on the first mirror. The same components as inFIG. 6 again receive the same reference number in FIG. 8. The freeworking distance A to the wafer is 60 mm in this embodiment, as it wasin the embodiment of FIG. 6, and thus it is greater than the useddiameter of the mirror next to the wafer, S5. Similarly, as with FIG. 6,the physical distance between the vertex V2 of the surface of mirror S2and the vertex V3 of the surface of mirror S3 was increasedsignificantly in comparison to that of U.S. Pat. No. 5,686,728, so thatlarge angles of incidence can be avoided in the system.

One difference to the objective of FIG. 6, is that in FIG. 8 theaperture stop B is placed on the first mirror S1. As a result of thisposition, a reduction in vignetting from the light reflected on S2 ispossible, whereas with the physical aperture stop positioned between S1and S2 light of the bundle of light rays running thorough the objectivecould pass above the aperture stop which is designed as a narrow ring.In the embodiment shown in FIG. 4, the aperture can be either an openingin the S1 mirror or an aperture disposed behind S1 close to this mirror.

Another advantage of this embodiment is the spherical design of mirrorS4, which presents advantages especially from the point of view ofmanufacturing, because mirror S4 is the largest mirror of the system.With such a design, the asphericity in the used range is increasedslightly to 10.5 μm. The largest angle of incidence occurs on mirror S5and is approximately 18.6°. The wavefront error of the arrangement is0.032λ within a 1.7 mm wide arc-shaped field at λ=13 nm. Furthermore, ifthe mirror S4 is designed to be slightly aspherical with 0.4 μm, thenthe wavefront error can be kept to 0.031 λ within a 1.8 mm widearc-shaped field at λ=13 nm. Efficient masking of the undesirable lightis obtained not only when the aperture stop is formed directly on mirrorS1, but also when it is arranged behind, i.e., after, mirror S1.Preferably, the aperture stop is positioned such that the followingrelationship is obtained:S2S1≦0.9×S2 aperture.

S2S1 denotes the mechanical distance of the vertex V2 of the surface ofmirror S2 and the vertex V1 of the surface of the mirror S1.

Table 11 shows the constructional data of the 5× objective according toFIG. 8 in Code V™ nomenclature, where the fourth mirror S4 is spherical.The mean radius of the 26×1.7 mm² image field is approximately 26 mm.TABLE 11 element No. Radius Thickness diameter type Object INF 85.2401256.1389 358.4668 S1 A(1) 0.0024 203.8941 REFL −358.4691 203.8845 0.0000201.9677 S2 A(2) 358.4691 201.9942 REFL aperture 60.7572 390.5456187.2498 0.0000 S3 A(3) −390.5456 188.9474 REFL −104.1273 505.8686 S4A(4) 494.6729 550.3686 REFL 114.3062 256.9217 281.6969 181.7337 S5 A(5)−281.6969 64.4286 REFL S6 A(6) 281.6969 187.8549 REFL 78.1545 Imageimage width 60.0041 53.6996 aspherical constants: Z = (CURV) Y²/[1 + (1− (1 + K) (CURV)²Y²)^(1/2)] + (A)Y⁴ + (B)Y⁶ + (C)Y⁸ + (D)Y¹⁰ asphereCURV K A B C D A(1) 0.00035280 −58.238840 0.00000E+00 2.14093E−152.29498E−20 0.00000E+00 A(2) 0.00097971 −4.160335 0.00000E+001.54696E−15 8.15622E−21 0.00000E+00 A(3) 0.00117863 −2.1364230.00000E+00 −1.78563E−16  3.45455E−20 0.00000E+00 A(4) 0.001243620.000000 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 A(5) 0.003388322.909987 0.00000E+00 7.90123E−13 7.04899E−17 0.00000E+00 A(6) 0.002786600.062534 0.00000E+00 2.79526E−16 7.00741E−21 0.00000E+00Reference wavelength = 13 nm

Another embodiment is shown in FIG. 9, where again the same referencenumbers are used for the same components as in the previous figures.Here, the aperture stop B is placed optically and physically on thesecondary mirror or second mirror S2. The ability to place the aperturestop on S2 makes manufacturing easier. Therefore this arrangement isadvantageous. The system of FIG. 9 is a 4× reduction system with awavefront error of 0.021 λ within a 2 mm wide image side arc-shapedfield at λ=13 nm. The maximum asphericity in the used range lies at 11.2μm, and the largest angle of incidence, which occurs at S5, isapproximately 18.3°. The ring field radius R as defined in FIG. 1 of thearc-shaped field in the image plane is approximately 26 mm, as with theprevious two embodiments. Furthermore, the distance between the imageplane and the vertex V5 of the surface of the mirror next to the imageplane, S5, is greater than the used diameter of the mirror next to thewafer, S5, and lies at around 59 mm in this embodiment.

Table 12 shows the optical parameters of the embodiment of FIG. 9 inCode V™ nomenclature. TABLE 12 element No. Radius thickness diameterType Object INF 84.0595 205.6642 473.5521 S1 A(1) −145.8261 147.3830REFL −327.7260 136.4700 aperture 112.0176 0.0000 S2 A(2) 473.5521112.1228 REFL 190.4830 163.5236 0.0000 184.4783 S3 A(3) −190.4830185.3828 REFL −399.1713 358.6720 S4 A(4) 589.6560 654.5228 REFL 207.5220310.1977 276.2668 175.3066 S5 A(5) −276.2668 65.2138 REFL S6 A(6)276.2668 182.8159 REFL 77.5085 image image width 59.0000 53.9968aspherical constants: Z = (CURV) Y²/[1 + (1 − (1 + K)(CURV)²Y²)^(1/2)] +(A)Y⁴ + (B)Y⁶ + (C)Y⁸ + (D)Y¹⁰ asphere CURV K A B C D A(1) 0.00015851441.008070 0.00000E+00 −3.49916E−16  1.27478E−19 −3.37021E−25 A(2)0.00089932 −5.032907 0.00000E+00 −6.95852E−15 −7.53236E−20 −2.74751E−24A(3) 0.00188578 0.913039 0.00000E+00 −1.60100E−15 −9.53850E−20 1.30729E−26 A(4) 0.00108147 0.038602 0.00000E+00  2.48925E−18−5.29046E−24 −4.37117E−31 A(5) 0.00269068 7.253316 0.00000E+00−5.70008E−13 −9.32236E−17 −6.09046E−21 A(6) 0.00281036 0.1509570.00000E+00  1.30822E−15  1.86627E−20  5.08158E−25Reference wavelength = 13 nm

FIG. 10 shows an embodiment of the invention which includes a fieldmirror S1, a first subsystem with the second to fourth mirror S2-S4 anda second subsystem with the fifth and sixth mirror, S5, S6. The fieldmirror S1 with imaging ratio, β, β>0 produces a virtual image of theobject in the object plane 2. The virtual image is then imaged by thefirst subsystem consisting of the second, third and fourth mirrors, S2,S3, S4, which has β<0, producing a real intermediate image Z in a planeconjugate to the object plane 2. The real intermediate image Z is imagedas a real image into image plane 4 by the second subsystem whichconsists of the fifth and sixth mirrors, S5, S6. The image sidenumerical aperture of the system is NA=0.28. The optical free workingdistance A between the vertex of the surface of the last mirror S5 andthe image plane 4 corresponds to at least the sum of one-third of theused diameter of the mirror nearest to the image plane and a lengthwhich lies between 20 and 30 mm. The aperture stop B is situated on thesecond mirror S2.

Table 13 shows the optical parameters of the embodiment of FIG. 10 inCode V™ nomenclature. TABLE 13 element No. Radius thickness DiameterType Object INF 151.2625 194.7605 229.0820 S1 A(1) −39.4068 162.9862REFL −189.6752 147.1426 aperture 65.0637 0.0000 S2 A(2) 229.0820 65.1650REFL 137.5708 168.3504 0.0000 230.5128 S3 A(3) −137.5708 234.0072 REFL−300.3445 386.2567 S4 A(4) 437.9153 630.7784 REFL 133.0981 343.1578353.0840 257.0225 S5 A(5) −353.0840 79.9521 REFL S6 A(6) 353.0840264.2853 REFL 78.6376 image image width 44.0000 54.0051 asphericalconstants: Z = (CURV) Y²/[1 + (1 − (1 + K)(CURV)²Y²)^(1/2)] + (A)Y⁴ +(B)Y⁶ + (C)Y⁸ + (D)Y¹⁰ + (E)Y¹² + (F)Y¹⁴ + (G)Y¹⁶ + (H)Y¹⁸ + (J)Y²⁰ K AB C D asphere CURV E F G H J A(1) −0.00080028 0.000000 −3.35378E−09 5.36841E−14 −7.86902E−19  −5.07886E−24  0.00000E+00 0.00000E+600.00000E+00 0.00000E+00 0.00000E+00 A(2) 0.00040002 0.000000 1.68187E−082.05570E−12 2.42710E−16 5.69764E−20 0.00000E+00 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 A(3) 0.00113964 −2.760663  0.00000E+00−3.55779E−15  1.03881E−19 −3.64996E−24  0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 A(4) 0.00128753 0.019273 0.00000E+005.82746E−18 −1.77496E−22  1.64954E−27 −6.20361E−33  0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 A(5) 0.00373007 11.68889680.00000E+00 −5.53902E−12  −4.32712E−16  −1.54425E−19  0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 A(6) 0.00240387−0.002567  0.00000E+00 −6.78955E−16  −8.39621E−21  −2.95854E−25 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00Reference wavelength = 13 nm

FIG. 11 shows a similar, yet alternative, embodiment to that of FIG. 10with a six-mirror objective with field mirror S1 as well as first andsecond subsystems as shown in FIG. 10. The embodiment shown in FIG. 11comprises as the embodiment in FIG. 10 an intermediate image Z.Furthermore the aperture B is formed on the second mirror S2 similar andthe numerical aperture on the image side is NA=0.30. The opticalparameters of this alternative embodiment are shown in Table 14 in CodeV™ nomenclature. TABLE 14 element No. radius thickness Diameter typeobject INF 103.2808 197.1874 219.3042 S1 A(1) −39.2890 157.6222 REFL−180.0152 142.1492 aperture 67.2659 0.0000 S2 A(2) 219.3042 67.4347 REFL131.2051 167.6895 0.0000 228.0182 S3 A(3) −131.2051 232.3162 REFL−247.5850 401.4441 S4 A(4) 378.7901 613.5493 REFL 134.4001 355.7774348.5086 268.3735 S5 A(5) −348.5086 81.5255 REFL S6 A(6) 348.5086269.2435 REFL 75.4983 image image width 36.1195 53.9942 asphericalconstants: Z = (CURV) Y²/[1 + (1 − (1 + K)(CURV)²Y²)^(1/2)] + (A)Y⁴ +(B)Y⁵ + (C)Y⁸ + (D)Y¹⁰ + (E)Y¹² + (F)Y¹⁴ + (G)Y¹⁶ + (H)Y¹⁸ + (J)Y²⁰ K AB C D asphere CURV E F G H J A(1) −0.00061615 0.000000 −5.19402E−09 1.09614E−13 −3.44621E−18  1.58573E−22 −7.07209E−27  0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 A(2) 0.00066911 0.000000 1.69112E−082.39908E−12 2.89763E−16 1.00572E−19 1.84514E−29 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 A(3) 0.00140031 0.000000 −8.71271E−10 −1.47622E−15  −3.40869E−20  4.32196E−24 −2.23484E−28  0.00000E+000.00000E+00 0.00000E+00 0.00000E+00 A(4) 0.00143731 0.000000 2.18165E+122.65405E−17 −2.01757E−22  1.14856E−28 1.49857E−32 −8.61043E−38 0.00000E+00 0.00000E+00 0.00000E+00 A(5) 0.00378996 0.000000 8.54406E−082.25929E−12 3.36372E−16 1.92565E−20 5.75469E−24 0.00000E+00 0.00000E+000.00000E+00 0.00000E+00 A(6) 0.00246680 0.000000 −3.61754E−12 −8.29704E−16  −1.53440E−20  −2.24433E−25  5.91279E−30 0.00000E+000.00000E+00 0.00000E+00 0.00000E+00Reference wavelength = 13 nm

FIGS. 12A and 12B define the used diameter D as used in the descriptionof the above embodiments. As a first example, the illuminated field 100on a mirror in FIG. 12A is a rectangular field. The illuminated fieldcorresponds to the area on a mirror onto which a bundle of light raysrunning through the objective from the object side to the image sideimpinge. The used diameter D according to FIG. 12A is then the diameterof the envelope circle 102, which encompasses the rectangle 100, wherethe corners 104 of the rectangle 100 lie on the envelope circle 102. Amore realistic example is shown in FIG. 12B. The illuminated field 100has a kidney shape, which is expected for the physical mirror surfacesof the mirrors S1-S6 or the so called used areas of the mirrors S1-S6,when the field in the image plane as well as the field in the objectplane is an arc shaped field as depicted in FIG. 5. The envelope circle102 encompasses the kidney shape fully and it coincides with the edge110 of the kidney shape at two points, 106, 108. The used diameter D ofthe physical mirror surface or the used area of the mirrors S1-S6 isthen given by the diameter of the envelope circle 102.

Thus, the invention provides a six-mirror projection objective with animaging scale of preferably 4×, 5× or 6× for use in an EUV projectionsystem. Other uses may be employed, however. The six-mirror projectionobjective has the resolution required for the image field, which is e.g.arc-shaped and has a advantageous structural design, since the aspheresof the mirror surfaces are relatively low, the angles of incidence ofthe rays of the bundle of light rays impinging the mirror surfaces aresmall, and there is enough room for mounting the mirrors.

It should be understood by a person skilled in the art, that thedisclosure content of this application comprises all possiblecombinations of any element(s) of any claims with any element(s) of anyother claim, as well as combinations of all claims amongst each other.

1-31. (canceled)
 32. An EUV optical projection system, comprising: afirst mirror, a second mirror, a third mirror, a fourth mirror, a fifthmirror, and a sixth mirror situated in an optical path from an objectplane to an image plane, for imaging an object in said object plane intoan image in said image plane, wherein said image has a width W and asecant length SL, and wherein said width W is greater than about 2 mm.33. The system according to claim 32, wherein said width W is betweenabout 2 mm and about 3 mm.
 34. The system according to claim 32, whereinsaid system has a numerical aperture greater than 0.18 at said image.35. The system according to claim 32, wherein said system forms anintermediate image along said optical path.
 36. The system according toclaim 35, wherein said intermediate image is formed between said secondmirror and said third mirror.
 37. The system according to claim 32,wherein said second mirror is concave.
 38. The system according to claim32, wherein said third mirror is convex.
 39. The system according toclaim 32, further comprising an aperture stop located along said opticalpath, between said first mirror and said second mirror.
 40. The systemaccording to claim 39, wherein said aperture stop is not located on saidfirst mirror, and not located on said second mirror.
 41. The systemaccording to claim 32, wherein said first mirror is concave, said fourthmirror is concave, said fifth mirror is convex, and said sixth mirror isconcave.
 42. The system according to claim 32, wherein said system has aRMS wavefront error of less than or equal to 0.017λ.